Ortho-nitroaromatic acids are used in many applications, for example, as intermediates in dyes, pharmaceuticals and agrochemicals. Ortho-nitrobenzoic acid is used as a corrosion inhibitor. Anthranilic acid (2-aminobenzoic acid) can be prepared by hydrogenation of o-nitrobenzoic acid. Anthranilic acid is used as an intermediate in the manufacture of dyes, pharmaceuticals, pigments, perfumes and flavors and as an antioxidant.
It is well documented in the literature that ONAC show great resistance to direct oxidation compared with meta- or para- nitroalkylaromatic compounds. This was reported by Hasegawa et al, Bulletin Chem. Soc. Jap., 51 (5), 1490 (1978), based on a study of the liquid phase autoxidation of nitroalkybenzenes catalyzed by a transition metal and bromide ions in acetic acid. More recently Partenheimer, Catalysis of Organic Reactions, Pub. Marcel Dekker, Inc. 1995, p. 314 stated, "m- and p- nitrotoluene can be smoothly oxidized to the nitrobenzoic acids in high yield, while o-nitrotoluene is completely inert." In U.S. Pat. No. 4,906,771, Young et al. state that "It has been known while meta and para- nitrotoluene are readily oxidized, by means of molecular oxygen in the presence of bromine and a heavy metal oxidation catalyst, to the corresponding nitrobenzoic acid the ortho isomer, namely o-nitrotoluene, is unexpectedly resistant to oxidation by this process. Similarly, other o-nitroalkylaromatics, such as 2-nitro-paraxylene, are unexpectedly resistant to oxidation." Young et al. were able to obtain 50 to 90 mole % yields of ortho-nitrobenzoic acid by the direct oxidation of a mixture of ortho-nitrotoluene and an equivalent amount of a co-oxidizable compound, such as toluene or para-xylene at 120.degree. C. to 220.degree. C. and 150 psig and 400 psig. This process suffers from the disadvantages of requiring by-product separation steps, decreased reactor capacity for the production of the desired ortho-nitrobenzoic acids, permitting no catalyst recycle and requiring the use of corrosion resistant equipment made from expensive alloys.
Hull, U.S. Pat. No. 2,673,217. describes a process for the selective oxidation of alkylaromatic compounds using aldehyde-activated catalysts, by first preparing a catalyst solution which is "activated" by simultaneously adding an aliphatic aldehyde and an oxygen-containing gas, and thereafter feeding the alkylaromatic compounds, together with an excess of oxygen, into the catalyst solution while maintaining the catalyst in the solution in an active state by continuously adding aldehyde. The oxidation is conducted at low temperatures (between 50.degree. C. and 90.degree. C.) and pressures (usually atmospheric pressure). Only meta and para-nitroalkyl aromatic compounds are described as substrates and yields of the corresponding aromatic acids are low, between 20% and 37%; there is no disclosure of the oxidation of ortho-nitroalkylaromatic compounds.
There is a need for a safe, environmentally acceptable and economically attractive direct oxidation process for the production of ortho-nitroaromatic acids in high yield from orthonitroalkylaromatic compounds.